1. Field of the Invention
A process for after-purifying a kieserite pre-concentrated electrostatically across a number of stages, is characterized in that the outside air used in the after-purification stage is adjusted by dehumidification in a conventional dehumidification plant to an absolute moisture of below 4.5 g/m.sup.3, and the kieserite mixture to be separated is triboelectrically charged with said air at a temperature of below 40.degree. C.
2. The Prior Art
It is part of the state of the art to isolate the mineral kieserite (i.e. MgSO.sub.4.H.sub.2 O) from hard salts of potassium deposits with the help of the dry electrostatic separation process. For this purpose, the crude salt, which is recovered by mining, is ground, screened to a predetermined grain size, provided with a small quantity of conditioning agent mostly of the organic type, and eddied with air of a certain temperature and humidity. In this way, a charge with the opposite sign is produced on the particles, and the mixture is separated in the electrostatic field.
The major part of the kieserite so obtained has fertilizer quality and is sold as such on the market, but considerable quantities are also processed further to potassium sulfate.
But a small proportion of the kieserite is also required with a higher purity. This mainly requires the removal of residual amounts of langbeinite (i.e. K.sub.2 SO.sub.4.2 MgSO.sub.4) and anhydrite (i.e. CaSO.sub.4).
DE-PS 33 34 665 describes an electrostatic process, according to which a langbeinite- and anhydrite-containing kieserite product is separated into a fraction rich in langbeinite and anhydrite, and a fraction with low langbeinite and anhydrite contents. According to the state of the art, a kieserite fraction is obtained from crude salt in a multi-stage electrostatic process. This fraction is chemically conditioned with 35 to 100 g/ton of parahalogen-benzoic acid or hydroxy-benzoic acid, and charged triboelectrically in a fluidized-bed heater at 55.degree. to 80.degree. C.
The separation takes place in an electrostatic freefall separator, which can be equipped with plate, belt or tube separators, at a field intensity of 4 to 7 kV/cm. The langbeinite- and anhydrite-rich fraction is collected on the positive electrode, and the low-langbeinite and low-anhydrite fraction on the negative electrode.
The drawback of this prior art process is that it has to be carried out at elevated temperatures, or that a chemical after-conditioning is required. Both requirements are very costly and may hinder the further treatment or use of the product produced.
Furthermore, it is known that the relative air humidity plays a decisive role in electrostatic separation. The relative humidity of the air (calculated in %), however, always depends also on the absolute moisture of the air (calculated in g/m.sup.3). Usually, the outside open air is used (for cost reasons) because it happens to be the cheapest source. However, in doing so, one depends on the value of the absolute moisture prevailing at a given time, and such value may vary greatly depending on the time of the year and the weather conditions. The only possibility of evading such circumstances is to control the relative humidity of the air via its temperature. In this connection, one has to take into account that with identical relative air humidity, the amount of water covering the surface of the salt decreases with the increase in temperature. This, however, is of decisive importance for triboelectric charging.